Field plates are used in high-electron-mobility transistors (“HEMT”) and metal-oxide-semiconductor field-effect transistors (“MOSFET”) to manipulate and reshape electric field distribution to reduce the maximum electric field of these transistors when high voltages are applied to the drain electrode. By reducing the maximum electric field, field plates increase the breakdown voltage and therefore achieve operations at higher voltages.
A comparison of FIG. 1 and FIG. 2 shows the benefit of a conventional field plate comprising metal. The transistor 100 in FIG. 1 comprises source 110, a gate 120, a drain 130, barrier layer 140 and body layer 150. The transistor 200 in FIG. 2 comprises a source 210, gate 220, drain 230, field plate 240, and barrier layer 250 and body layer 260. In typical Si MOSFETs, oxides such as SiO2 or other dielectric materials are deposited onto a Si surface (i.e., body layer 150 or 260). A conducting channel 190 typically is formed at SiO2/Si interface through a process called inversion by which the mobile carriers are induced by the gate voltage. For the case of GaN HEMT, body layers 150 and 250 may comprise gallium nitride and barrier layers 140 and 260 may comprise aluminum gallium nitride and/or additional dielectric material. Referring to FIG. 1, in response to the gate voltage, electric charges 180s are induced at the interface between barrier layer 140 and body layer 150. Because typically both barrier layer 140 and body layer 150 are free from intentionally doped impurity, the mobility of the electrons is higher than that of conventional n-type semiconductor. For illustration purpose, in the following sections, n-channel AlGaN/GaN HEMT is described below but one of ordinary skill in the art would understand that other types of HEMT or MOSFETs operate in similar fashion except the exact location of charges may be different.
Regarding FIG. 1, when a voltage higher than saturation voltage is to transistor 100, electric charges 180 appear in the transistor 100 within a conducting channel 190. The gate 120 usually comprises metal and certain negative charges 160 are induced at the interface between the barrier layer 140 and body layer 150. Due to the geometry and metal composition of the gate 120, electric fields 170 from these certain negative charges 160 concentrate at the corner of the gate 120.
A conventional solution to reduce the concentration of the electric field at the corner of the gate 120 is the use of a metal field plate 240, as shown in FIG. 2. The field plate 240 is another metal layer that extends over the gate 120 but is positioned slightly further away from the conducting channel 295. When a voltage higher than saturation voltage is applied to transistor 200, electric charges 290 appear within the conducting channel 295. One of ordinary skill in the art would understand that the electric fields 270 generated from electric charges 280 by an applied voltage are not limited to only those shown in FIG. 2. The electric fields 270 shown are representative of the localized electric fields generated from the certain electric charges 280 that concentrate along the bottom surface and at the corner of the conventional field plate 240.
Rather than concentrating at the corner of gate 220, the electric fields 270 are distributed along the bottom surface of the field plate 240. By preventing the electric fields 270 from concentrating at the corner of the gate 220 and redistributing the electric fields 270 to the bottom surface of the field plate 240, the field plate 240 increases the distance of the electric fields 270 from the conducting channel 295. As a result, in comparison to a transistor 100 without a field plate, the breakdown voltage of the transistor 200 with a field plate 240 is increased.
Conventional field plate designs consist of layers of metal. Metal field plates, while achieving a reduction in electric fields at lower voltages, exhibit the same problem of localized concentration of electric fields as the applied voltage increases. In other words, as the electric charges 290 increases (as a result of the increased voltage) within the conducting channel 295, electric fields 270 concentrate at the corner of the field plate 240. Therefore, the effectiveness of the field plate 240 is diminished as the applied voltage increases, which limits the application of the field plate 240 for high-voltage applications. These problems have become particularly prevalent with any high-voltage transistors such as gallium nitride HEMTs and silicon MOSFETs.
To remedy the localized electric field at the corner of the metal field plate, conventional solutions have included adding a second field plate. Examples of this solution can be found in the following references: Saito et al., “Current Collapseless High-Voltage GaN-HEMT and its 50-W Boost Converter Operation,” and Xing et al., “High Breakdown Voltage AlGaN—GaN HEMTs Achieved by Multiple Field Plates,” both of which are incorporated herein by reference. These references disclose conventional field plates where the second field plate also is made of metal and therefore, the electric field still develops at the corners of the second field plate. The electrical field along the channel of a conventional GaN HEMT device generally exhibits sharp peaks at the corners of the metal structures such as the end of the gate metal or the end of field plates. Moreover, adding additional field plates increases both complexity and cost of manufacturing.
The reason for the formation of peaks at the corners is explained by the electric charge crowding properties for metal. These electric charges are induced by the position charges in the channel when the high drain voltages are applied to the drain contact. The positive charges appear as the result of the depletion of two-dimensional electron gas (2DEG). The high electric field at the corners of the field plate increases the risk of voltage breakdown of the transistor.
Because of charge crowding effect at the boundaries of the gate and metal field plates, conventional metal field plates present sub-optimized solutions no matter how many conventional field plates are used. Moreover, while increasing the coverage of the field plate over a region allows for higher voltages to be sustained without reaching the breakdown field, this increased coverage also increases the distance between the drain and the gate, which increases on-resistance.
Previous attempts to improve upon conventional field plates have relied on increasing the distance between the electric charges. Voltage is developed by electric field over distance. E=−∇V. And, electric field is originated from charge:
      ∇          ·      E        =      ρ    ɛ  (Poisson equation, where ρ is charge density and c is the permittivity of the material). Thus, one solution to reduce the electric field with as voltage increases involves increasing the distance between corresponding positive and negative charges, i.e., moving the electric field away from the channel.
Another recent approach is the concept of gate-connected slant or sloping field plates to increase the distance between the field plate and the semiconductor substrate. The increased distance reduces the peak electric field near the conducting channel for high voltage breakdown operation. While slanted structures offer one solution over conventional field plates, such structure are difficult to fabricate using traditional lithography processes.
However, the problem with any solution using conventional metal field plates is that metal is known to have high free electron density and therefore provides charges near the surface of the metal. With any metal field plate, a high density of charges accumulate near the corner of the metal field plate. Therefore, there has been a long felt need for a solution to these issues to reduce the maximum electric field of these transistors when high voltages are applied without further complicating the manufacturing process.